When there are enhanced safety requirements, electronic locking systems which operate by means of electromagnetic waves are used. In particular, in the case of motor vehicles, such locking systems are used as door locking systems for providing access authorization and/or as ignition lock systems, steering wheel locks, immobilizers or the like for providing authorization to drive.
Such locking systems are known from DE 43 40 260 A1. The locking system is composed of a first device which has at least two states and is embodied as a control device for unlocking and/or locking the car doors, the ignition lock or the like, and an associated second device which is embodied in the manner of an electronic key. The two devices have, for their intended mode of operation, transmission means and/or receiving means for electromagnetic signals with frequencies ν lying in a frequency band between cut-off frequencies νa, νb. In the intended mode of operation, at least one signal for authenticating the key can be transmitted as a coded operating signal between the key and the control device so that after a positive evaluation of the transmitted operating signal, and thus when the key has been authorized, it is possible to bring about a change in the state of the control device.
Such locking systems have also been further developed with what are referred to as “keyless” functionalities. Thus, in the case of the “keyless entry” functionality, manual activation of the electronic key by the user is no longer necessary. It is sufficient for the user to carry the key on his person. The operating signal is then transmitted automatically between the two devices for the access authorization if the user is located in an active range in the vicinity of the motor vehicle and activates, for example, the door handle on the car door there. Likewise, these locking systems can have a “keyless go” functionality, with the operating signal for providing the authorization to drive being transmitted automatically between the two devices for the case when the user is inside the motor vehicle and activates, for example, a start/stop pushbutton key on the dashboard.
In order to transmit signals between the key and the control device, a specific setpoint frequency νs is provided for the respective locking system. However, owing to component tolerances and tolerances in fabrication, in practice deviations from the setpoint frequency νs occur at the transmission means in the two devices. Subsequent adjustment of the two devices to the setpoint frequency νs is complex, associated with high manufacturing costs and increases the rejection rate.
In addition, with the key, in particular a small key with a keyless functionality, adjustment to the setpoint frequency νs is not possible in all cases. For this reason it is necessary for the receiving means in the control device to search through the permissible frequency range [νa, νb], specifically to carry out a narrowband search owing to the desired large range for the transmission of signals in order to then set the particular actual frequency νi of the transmission means of the key for the transmission of signals. As a result of this the transmission of signals may take a considerable time so that the locking system is felt to be inconvenient to operate by the user of the motor vehicle.
The time problem becomes even more acute if a plurality of keys, each with different actual frequencies νi, are associated with the same control device. In addition, a plurality of keys may be located in the active range of a motor vehicle, which is equipped with keyless functionalities, and these keys may be associated with the motor vehicle but also with another keyless motor vehicle, which further increases the time required to search through the entire permissible frequency range [νa, νb].